Demodulation circuit



May 26,1942. E. PETRsoN 2,284,444

DEMODULATION CIRCUITS Filed Aug. 27, 1940 L Osc 4 l LIM/TER c, L, En /l//2 HNL Moa "E 4M? 7??. CJ AEE me Re: /7 f \a 5 6 9 R3 F IG. 5

NoN-WEAR I I I mmm/c6 d \/d bw PETERSON A TTOR/VEV Patented May 26, 1942UNlT-ED STATES PTENT, OFFICE I DEMODULATION CIRCUIT Eugene Peterson, NewYork, N.' Y., assignolrto Bell Telephone Laboratories, Incorporated,V.New York, N. Y., acorporation of New York Application August 27, 1940,Serial No. 354,351

(Cl. Z50- 20) 9 Claims.

This invention relates to detection or demodulation of wave-lengthmodulated waves, that is, frequency modulated or phase modulated waves,in high frequency signals and more particularly to'circuit arrangementsyin which use is made of the properties of saturated core-inductances ofthe type described in'patent to Wrathall 2,117,752, May 17,1938. n

The purpose 'of the invention is to provide a means for securing afrequency indication or a phase indication and Vto this' end I providemeans whereby pulsesare'generated, the frequencv of which is determinedsolely by thev momentary frequency for frequency modulated waves,'or thetime displacements of which from normal or vno signal condition isproportional to vthe momentary phase displacement in phase modulatingwaves. In order at the same time to render the received signal dependentsolely on frequency or on phase modulation, use is made of amplitudelimiting devices all in a manner hereinafter described. Y

The invention will be better understood by reference to thefollowingspeciiication and the ac-l companying drawing, in' which:

Fig. 1 isa diagram of a circuit adapted for demodulation or detection ofaY frequency modulated wave;

Fig. 2 discloses a modification of a part of the circuit of Fig; 1;

Fig. 3 shows curves for explaining 'the principles involved inthecircuit of Fig. 1;

Figs. 4 to 6 are modifications rof a detail of Fig. 1;

Fig. 7 shows a circuit adapted for detection or demodulation of a phasemodulated wave;

Figs. 8 and 9 are curves for explaining the principles involved in thecircuit 'of Fig. 6; and

nected 'to themodulator to passthis intermediateV frequency-anditsside-b'ands, and Vthe output of `the vband-pass lter is thenimpressed on a circuit containing the non-linear magnetic coil 1. Thiscircuit is ofv the..type described inthe patent to Wrathall referred ktoabove and possesses the property of generating sharp impulses of veryshort duration with onepulse for each positive loop and one .in thereversedirection for each negative loop of the impressed wave. This is.illustrated in Fig. 3 which shows schematically a short section of'afrequency modulated wave, The generatedimpulses, also shown in thisfigure, occur very nearly at the timewhen the current wave inv the coilis passing through itszero value. It is apparent that were one tointegrate the effect of all of these pulses the resultwould besubstantially Zero. In accordance with one form of myV invention,however, I connect across the coil 1 a rectifier element 9 whichsuppresses the impulses of the one sign and the` g integrated value ofthe Vremainder willbe rep- Fig. l0 illustrates the generation' ofcontrol waves and transmission thereof to atran'smitting and a receivingstation.

Referring more particularlyto Fig. 1, there is shown an incoming circuitfor a received frequency modulated wave, this incoming circuit being anantenna system, as shown, or an appropriate line circuit. The incomingwave Vmay be impressed directly on a harmonic generator coil I or it maybe first impressed on a modulator circuit 3 vto which is also connectedan oscillator 4 of controlled frequency, the frequency resented by a lowfrequency current in a translating device l'l, the amplitude of thecurrent being proportional to, the Ynumber of pulses generated per .unitof time and thus proportional to the momentary frequency of the originalincom- In another form `of my invention I use .bothpulses by thefull'wave rectifier of Fig. 2. Obviously the output voltage across theresistance R3 will contain high frequency components. These may befiltered out so far as the translating device I7 is concerned by alow-pass nlter Il. Appropriate amplifiers, such as 6 and I2, maybeinserted as desired. l A In a frequency modulated wave it is desirablethat there shall be no amplitude modulation present. Due to variousimperfections, however, in the circuitfor in thev transmitting medium,variations in amplitude may enter. These effects largely disappear inusing coil 'I but not entirely so,.for itis found that the amplitude ofthe peaks generated by the coil is substantially proportional to thefour-tenths power of the input.

In order to more fully. eliminate the corresponding disturbances I findit desirable in some instances to associate withthe circuit someamplitude limitingdevice such` as that shown at I4. While such alimiting device may be introduced at anyof various points in thecircuit, I have shown it as bridged across the condenser C3. In

this case the limiting device may take-on a variety of forms such asshown in Figs.,4 to 6. For example, in Fig. 4 the limiting device is`shown 1S WO diode rectifiers which will substantially prevent thevoltage across condenser C3 from rising above a specified value. In Fig.5 the limiting device is shown as a thyrite resistor, and in Fig. 6 itis shown as two oppositely directed copper-oxide rectiers connectedA inparallel and j biased by suitable voltages so thatV they serve as`virtually open circuits until the voltage over C3 rises above aspecified value. Y Y In one mode of operationY ofmagnetic harmonicgeneratorsthe condenser C3 is made very large and R3 is made solargethat it is virtually an open circuit, in which case practically nov'oltage is built up across the condenser and practically no currentflows through R3 so that voltage across R3 is substantially .thesame asthat across the non-linear coil L. Thisarrangement, which may be spokenof asv open vcircuitopera tion, is found particularly advantageous ifthe an unmodulatedarriving wave occur at the time when the locallygenerated wave as applied to demodulator I3 is passing through its zerovalue. Under these conditions the modulation effects, which arevproportional to the instantaneous cross-product of the two inputs, aresubstantially equal tozero.

If, however, the pulses are dis- Y placedrin time due to phasemodulation, then circuit into which the coil works has a high in;

put impedance. For the suppression. of .the pulses of one. sign,'.ifsuch suppression visqused,

therectier maybe placed. in series with R3 or in parallel; fIneitheryoasve yI findthat the limiter forf the purposes described aboveisgnotV required, for vifthe instantaneous amplitudel of the signalincreases due to such causesasjnoise, the magnetizing force in thecoilrises more rapidly giv ing a hgherreactance voltage butit Vsweeps overthe range of high vinductanceof theY coill in a shorter time, that i,s,tl1,eV height ,of the generated peakis somewhat greater .but itsduration is. less -so that the area under the peakloop is substantiallyindependent ofthe amplitude of the-sig-y I nal; It is, these areas whichare integrated inthe translating device. :Another way to express thisisv to note .thatunder these conditions theaveragecurrent through R3will be the same no matter what the incoming wave amplitude' may be solong as it is above a certaink minimum value. Thus underthesecircumstances. the non-linear coiland itsassociated output currentfunctions asaunitary self-limiting detecting device for a frequencymodulated wave. [In all cases and .es-

pecially the ,one inwhichthe rectifier isl placed in series with Rz'it.is ydesirable that the rectifier shall bealinear onefor `thatit shall beoperated in that part-,forwhich it is substantially linear. j My`circuit'arrangement for detectionor. ciemodulationof aqphase modulatedwave .is illus?. trated in Fig. 7. Here again the incomingV phasemodulatedwave received over line 2U from distant transmitter I9 isimpressedon a harmonic the demodulation cross-product term isproportional to the instantaneous'value of the locally introducedcarrier, and thus the amplitude of the resultant low frequency wave isproportional Ato the momentary phase modulation of the incoming wave. Inorder to provide the precise agreement-between the incoming carrierfrequencyfa'nd thatproduced at the receiver some of the wave from sourceI5 may be supplied over line I8 to the distant transmitter I9 to serveas theV transmitting carrier wave from station I 9.

These facts may be readily shown by thefollowing'w'analysis in whichisa-is the' phase dispiacmentatjany m"- This'alon'g'with the locallyintroduced carrier is impressed onthe demodulating elementso that Intheoutputof thedemodulator theA terms'of interest are primarily thecross-product terms generator 1. bridged. by a rectifier element 9,which latter suppresses impulses of one sign., In

Fig. 8 the impulses are shown in full lines and as .equally spaced,corresponding tothe generation, of the impulses when the Aincoming waveis' notsignal modulated. If, ontheother hand,

Of these, all the alternating current 4cross-product termsare ofcarrier'frequencies withftheir side-bands or har'mbnics thereof exceptthe one corresponding to n`=1'. This term is the one'low frequency termand appears *as 'A Y J Ilzsin (KQ dos qu Furthermore, if lthe phasedisplacement is relathe, incoming wave isY phase modulated, then .the

timing of the impulses will be altered forward or balgward fraccordancewith .the phase of the modulated..wave. as Yshown by .the dottedimpulses.. y

Itis apparent that the mere integration of thesenew. pulses will not'initself give rise in a translatingdevice toany appreciable signalcurrent., In the circuit.arrangement, however, I impress the pulses on amodulating element I3 on whichthere is also impresseda carrier frequencyp/21r from a source I5. This carrier frequency must be vprecisely'thatof the incoming carrier frequencyr and is -shown by the sinusoidal curveof Fig, 8. The phase of ythis locally introduced carrier frequency asapplied tothe demodulator I3 is` so adjusted that kthe pulses fromtively small, that is, if KQ cos qt is small, this then becomes y'@(KQcos qt- I s 2 24 showing that the low frequencyoutputwisrsubstantially u linear with respect to the original modulatingwave, thus giving a good reproduccos Sgt-I4 The case thus treatedisaspeciai one but it Y may be generalized. If, .for example, thelocallyintroduced carrier is some harmonie of the frequency p/21r, such as thekth harmonic, then the input on'the modulator will be given by l[Ebn'cos n pt+ l sin lc pt The only cross-product term which yieldssignal 2i which may lcontain apparatus similar to that frequency will bethat for which n is equal to 7c and this term appears as j A doubleadvantage now becomes apparent. The first of these is the fact that thenumber of pulses per unit time will be increased by .the factor lc, thusincreasing the strength of signal. The second is that the factorcorresponding to the depth of modulation, or frequencyl swing, has-`been increased from KQ to kKQ, thatis, has been `increased by' thefactor Ic, representing a further increase in signal strength.

It should be apparent' from inspection of .Fig '7 that the range ofphase modulation over which the final output is substantially linearwith respect to the original signal is that over which the instantaneousvalue of the locally introduced carrier frequency is substantiallylinear, that is, in the neighborhood of the cross-over points in thecurve of Fig. 8. As sine departs from linearity, the distortionincreases. It is possible, however, to increasevery materially the rangeof linearity by altering the form of the locally introduced carrierfrequency. This is illustrated at l,Y 9,- 14 and V|3'of Fig. '7. fAcarrier wave of frequency 'p is supplied to station I9` over path Y i8from a suitable source asinV Figfl. For the in Fig. 9, which makes useof certain disclosures in copending application of Manley-WrathallSerial No. 354,372, filed August 27, 1940, and consists in introducingin place of a sinusoidal carrier wave, a saw-toothed wave, as shown inFig. 9. Any suitable method for obtaining such a saw-toothed wave from asine wave is satisfactory, such as a relaxation circuit, and While ringeneral it will not be physically possible to duplicate the ideal curve,any step towards this will permit an extension of the amount of phasemodulation without appreciable departure from linear output. It shouldbe noted that there would normally have been an impulse corresponding tothe points b in Fig. 9, as well as the points a, but any disturbancesdue to these are eliminated by the rectifier element across thenon-linear coil 'I whereby all pulses of one sign are suppressed.

It will be found from the above analysis that the best condition underwhich a cross-product term of signal frequency is obtained is when thelocally introduced carrier frequency is 90A degrees out of phase withthe incoming carrier frequency. Thus, since the incoming carrierfrequency in the expressions above has been represented by a cosinefunction, the locally introduced term must be represented by a sinefunction. This brings out the necessity, as has already been referred toabove, of properly phasing the local carrier frequency with respect tothe incoming carrier frequency. The importance is also apparent ofmaintaining the locally generated frequency precisely in step with theincoming carrier frequency. In case my circuits are being used inconnection with an extensive wire system, then it is evident that thecarrier frequency to be locally introduced may be derived from a sourceof standard frequency which supplies this standard frequency to thetransmitting station also. If such derivation of the locally introducedcarrier is not feasible, then it may be obtained by radio transmissionfrom some suitable station of two frequencies, the difference of whichis equal to the desired carrier and is obtained by suitable demodulationcircuits.

This is illustrated in Fig. 10 in which the transmitting station I9 atline 20 may be as in Fig. 7, leading in this case to a receiving stationpurpose of supplying a local carrier wave to station 2l of propervfrequency and properly phased with respect to the incoming carrierfrequency, radio transmitter 22 is provided. Thisfsends out twofrequencies Fi and F2Y whose difference frequency is equal to thefrequency p. 'Ihe two frequencies F1 and F2 are received in radioreceiver 26, demodulated at 21 and the difference frequency p issupplied over path I6 to the receiving station 2|. Y

What is claimed is: i `Y l. In a receiving system for wave-lengthmodulated waves, a demodulation circuit comprising a saturable coremagnetic harmonic generator adapted to `produce sharp positive andnegative pulses, means for impressing thereon wave-length modulatedwaves of sufficient amplitude to cause saturation of the core of saidharmonic generator for a part of each cycle to generate alternatepositive and negative pulses timed in accordance with the received wave,means for suppressing pulses of the one sign whereby pulses of one signremain variably spaced on a time axis in accordance with the wave-lengthmodulation of the received waves, and translating means for producinglow frequency output current varying in amplitude in accordance with thevariable spacing of said pulses on the time axis.

2. In a demodulation 4circuit for phase modulated waves, a magneticharmonic generator eX- cted by received phase modulated waves forgeneration of positive and negative pulses, means for suppressing pulsesof one sign, a demodulation element in the circuit on which theremaining pulses are' impressed, and a source of unmodulated carrierfrequency associated with the demodulating element, the output of thedemodulator for each pulse being substantially proportional to its phasedisplacement with respect to its own carrier frequency.

3. In a demodulation circuit for phase modulated waves, a magneticharmonic generator excited by received phase modulated waves forgeneration of positive and negative pulses, means for suppressing pulsesof one sign, a demodulation element in the circuit on which-theremaining pulses are impressed, a source of unmodulated carrierfrequency associated with the demodulating element and so phased thatthere is substantially no demodulation product from pulses due tounmodulated waves but for other pulses the demodulation product issubstantially proportional to their phase displacements.

4. The combination of claim 2, characterized by this, that the source oflocally introduced unmodulated carrier frequency yields a saw-toothedwave whereby the demodulation output is linear with respect to phasedisplacement over a wider range.

5. The combination of claim 2, characterized f in this that thelocallyintroduced unmodulated carrier frequencyY is the kth harmonic of`the incoming signal carrier frequency.

4modulating element alocally introduced carrier A frequency which is thekth harmonic of the signal carrier frequency, and deriving from theoutput of the demodulatorthe signalwwave.

7. The combinationof claim 6 further characterized in this that themethod includes so shaping the locallyintroduced carrier frequencycapacitance and. a large resistance in series and means to derive thesignal from the generated pulses.r y Y 9. In a receiving system forwaves the frequency ofwhich has been modulated in accordance withsignals, a. demodulation circuit comthat it is saw-toothed in formwhereby theregion of linear demodulation is extended. A

8. In 'a receiving system for waves `the frequency of which has beenmodulated in accordance with signals, a demodulation circuit comprisinga magnetic harmonic generator acting as a unitary self-limiting pulsegenerator, the generator including a non1ine`ar,saturating core magneticcoil on which the received frequency modulated wave is impressed, a loadcircuit shunting the non-linear coil comprising a large

